The present invention relates to technology for data storage.
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Non-volatile memories formed from reversible resistance-switching elements are also known. For example, U.S. Patent Application Publication 2006/0250836, published Nov. 9, 2006, and titled “Rewriteable Memory Cell Comprising A Diode And A Resistance-Switching Material,” incorporated herein by reference, describes a rewriteable non-volatile memory cell that includes a diode coupled in series with a reversible resistance-switching material such as a metal oxide or metal nitride. These reversible resistance-switching materials are of interest for use in nonvolatile memory arrays. One resistance state may correspond to a data “0,” for example, while the other resistance state corresponds to a data “1.” Some of these materials may have more than two stable resistance states.
Moreover, various types of volatile memory devices are known, such as DRAM. Further, memory devices can have one layer of storage elements, or multiple layers in so-called 3-D memory devices.
Each memory device is typically tested before being shipped to the end user to identify defective storage elements which are not suitable for storing data. Due to manufacturing variations, a number of such bad storage elements are inevitably identified. Each bad storage element can be marked to prevent access to it. Typically, a number of additional, redundant storage elements are provided for use in place of the bad storage elements. In a column redundancy (CR) approach, a column of storage elements which has one or more bad storage elements is replaced by a redundant column. Column redundancy data which identifies the bad columns and the respective redundant columns can be stored in the memory device. When the memory device is powered up, the column redundancy data is transferred to a working memory location of the device, such as CAM (Content Addressable Memory). However, the size of the CAM is proportionate to the amount of redundancy information which is stored for the memory device. As a result, precious space in the memory device is consumed by the column redundancy data stored in the volatile memory (CAM).
Techniques are needed for reducing the amount of space in a memory device which is needed to store column redundancy data, while still maintaining the same redundancy coverage.